According to a new study, smiles originated at least 30 million years ago, when monkeys of the old world and our direct ancestors diverged. This research discovered that when human and chimp infants are dozing, they sometimes show facial movements that resemble smiles.
Researchers suggest that such facial expressions, called spontaneous smiles, seem to be evolutionary origin of our real smiles and laughter. They proved that this not only happened to high-order primates like humans and chimps but also in newborn Japanese macaques, which are more distant in the evolutionary tree.
Masaki Tomonaga from Kyoto University, Japan, said: “About a decade ago we found that chimp infants also display spontaneous smiles. Since we see the same behaviour in more distant relatives, we can infer that the origin of smiles go back at least 30 million years ago, when old world monkeys and our direct ancestors diverged.”
More information here.